Varifocal objective with movable lens component of positive refractivity



omnun ,nw

3,466,1 l3 ENT p 9, 1969 L. BERTELE VARIFOCAL OBJECTIVE WITH MOVABLE LENS COMPON OF POSITIVE REFRACTIVITY Filed Aug. 24, 1964 INVENTOR Luna (G 66972145 BY M ATTORNEY- United States Patent U.S. Cl. 350-184 2 Claims ABSTRACT OF THE DISCLOSURE A variable focal length objective of Simple physical construction affording an aperture ratio of l: 1.4 or greater and an image forming power that meets high requirements. The objective comprises at least 6 lens members, the second lens member being displaceably mounted so as to change the focal distance. With respect to the side of the longer conjugate, the first'lens member of the objective has a dispersing or negative power, the second and third lens members having a positive or collective power, the lens surface of the fourth lens member being convexly formed to such a degree that the radius of curvature of said lens surface is shorter than the radius of curvature of the preceding glass-air surfaces. The first lens member is further shaped in such a fashion that the stronger curved surface thereof exhibits a dispersing or negative power and is curved towards the second lens member. Regardless of the position of the displaceable second lens member, the focal distance of the first two lens members taken together is always positive.

The present invention relates to an objective with a variable focal length, for example, for exposure and projection purposes.

In accordance with the invention there is provided an objective of variable focal length comprising at least six lens members, wherein, considering the lens members from the object side, which is the side of the longer conjugate of the objective, the first has a negative power, the second is displaceable and has a positive power, the third has a positive power and the fourth has a surface with convex curvature, which surface is the nearer surface of the fourth lens member to the third lens member and has a radius of curvature which is shorter than that of the air-glass surfaces of the preceding three lens members.

Independently of the position of the second lens member, the focal length of the first two lens members taken together is always positive.

Objectives are known to the applicant having a different sequence of refractive powers in which, regarded from the side of the longer conjugate, the first lens member has a positive power, the second displaceable member has a negative power and the third fixed member again has a positive power. .The focal length of the first two lens members is in any case negative. With this objective, the. image after the second lens member is therefore virtual and, thus, is on the object side of this lens. This position of the image places an increased demand on the further lens members as regards their refractive powers. However, if there is chosen for the first lens members an arrangement which produces a real image position after the displaceable lens, some of the following lens members are considerably relieved. In this way, the production of a large aperture ratio for the entire system is simplified and it has been shown by investigation that the weakening of the refractive powers of individual lens members provides a considerable improvement in the image-forming power of the system. These advantages can be produced it the first lens member, turned towards the object, has a negative or dispersive power, the second displaceable lens 3,466,] 13 Patented Sept. 9, 1969 member has a positive or collective power, the third lens member has a positive power and that lens surface of the fourth lens member nearest the third lens member is made convex. The further design of the fourth and the following lens members will be in accordance with optical requirements.

In order to obtain the best possible conditions as regards correction in each position of the second lens member, it is further possible to ensure that, in the first lens member, the more strongly curved surface having a negative or dispersing action is curved towards the second lens member. The curvature of this surface can be so strong that the first lens member becomes a meniscus curved towards the second member. If the following positive or collective lens member is constructed as a simple lens, this lens is expediently of a bi-convex form.

The focal length of the first lens member is so chosen that this focal length is less than 9 times and greater than 2.5 times that of the free lens diameter which is used. The focal length of the displaceable second lens member is smaller than 6 times and greater than 1.5 times that of the necessary free lens diameter.

Should the transverse color errors occurring to a slight degree with the displacement of the collective lens be disturbing, both or only one of the first two lens members can be formed as cemented members and can also be subdivided in two or more single lenses.

The further design of the fourth lens member and the following lens members can vary. A preferred lens sequence is formed, because of its simplicity, by a positive or collective lens as fourth lens member with its convex surface facing the object, by the fifth lens member as a negative or dispersive lens of which the more strongly curved surface faces the image, and by the sixth lens member as a positive or collective lens.

In order to produce the best possible zonal correction of the spherical aberration, it has proved to be advantageous to make the radius of the strongly convexly curved surface of the fourth lens member less than half the focal length of the lens group consisting of the third, fourth, fifth and sixth lens members.

For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawing in which the single figure illustrates an objective of variable focal length and large aperture ratio for exposure and projection purposes.

In the figure L is a negative or dispersing first lens member, L is a displaceable positive or collective second lens member, while L L L and L are the other lens members.

The symbols r represent the radii of curvature, the symbols d represent the thicknesses of the lenses and the symbols l the air spaces. The symbol n represents the refractive index for the helium line, and the symbol v represents the Abbe number.

The focal length F of the entire system with the middle position of the lens member L, is 19.8 millimeters, while with a displacement of the lens member L: by 15.7 millimeters towards the rear, the focal length is 15.65 millimeters. With an equally large displacement of the lens member L towards the front, the focal length is 24.7 millimeters. The focal length of the lens group L to L is 22.95 millimeters and the radius r =9.819 millimeters that is to say, 42.8% of this focal length.

The focal length of lens member L is -120.5 millimeters and the free diameter is about 24 millimeters.

The focal length of lens member L, is +68.l millimeters and the free diameter about 24 millimeters.

The focal length of lens members L, and L varies depending upon the position of the second lens member L, from +151 millimeters to millimeters.

EXAMPLE nd U T; =39-6 l H fir-=08 1. 5111 63. 4

l =16.O:l:15.7 r =+84.0 L: dz=2.7 1. 62041 60. 3

Z2 =16.0=F15.7 7'5 =+35.0 L d3=1.8 1. 62041 I30. 3

T =0.15 r1 +9.81% Lt d4=3.3 1. 62041 60. 3

1; =05 T9 =+400.0 L d5=4.8 1.7618 27.0

l =3.16 r ==+14.9fi28 L dg=2.7 1.6910 54.8

ao=7.74; F=l9.68 in middle position of L1.

What is claimed is:

1. An objective of variable focal length comprising at least six lens members, wherein, considering the lens members from the side of the longer conjugate:

(a) the first lens member is a dispersing meniscus which is curved toward the second lens member; (b) the second lens member is a displaceable bi-convex lens member of positive power;

(c) the first and second lens members together have a positive power;

(d) the third lens member has a positive power and the convex surface facing the second lens member is more strongly curved than the lens surface facing away therefrom;

(e) the fourth lens member has positive power with a convex curved surface facing the third lens member;

(f) said convex curved surface of said fourth lens member has a radius of curvature which is shorter than that of each of the air-glass surfaces of the first three lens members;

(g) the fifth lens member has a negative power, with the more strongly curved surface facing the image; and

(h) the sixth lens member is bi-convex and has a positive power.

2. An objective according to claim 1 wherein:

the focal length F of the entire system ranges from 15.65 mm. when said second lens member is in its rearmost position to 24.7 millimeters when said second lens member is in its forward position and is 19.68 millimeters when the second lens member is in its middle position;

the focal length of the lens group consisting of said third, fourth, fifth and sixth lens members is 22.95 millimeters;

the focal length of said first lens member is -120.5 millimeters and the free diameter thereof is about 24 millimeters;

the focal length of said second lens member is 68.1 millimeters and the free diameter thereof is about 24 millimeters;

said first through sixth lens members, designated L L respectively, have radii of curvature r 4 thicknesses d d and separations -1 respectively, said radii of curvature, thicknesses and separations, as well as the respective refractive indices n and Abbe nlgmbers v are substantially as given in the following ta le:

r1 =39.6 L1 d1=0.8 1.5111 63.4

Zi=16.0:t15-7' r; =+84.0 L: d2=2.7 1 .6204]. 60 .3

l2 =16.0=F15-7 T5 =+35.0 L3 d3=1.8 1.62041 .3

1;; =0.15 f1 =+9.8130 L4 d4=3.3 1.62041 .3

l =0-6 I: =+400.0 L5 d5=4.8 1.7618 27 .0

l5 =3.16 m=-l 14.9628 143 du=2.7 1.6910 54.8

References Cited UNITED STATES PATENTS 3,227,042 1/1966 Smejkal et al. 350l84 3,274,887 9/1966 M86116! 350-484 3,294,470 12/1966 Dakin 61 al. 350184 JOHN K. CORBIN, Primary Examiner US. Cl. X.R. 

